In process control industry applications, many control systems vent to atmosphere and are generally very noisy because of the high pressure ratios and high exit velocities involved. A properly designed vent, in combination with a properly sized valve, can reduce the overall system noise level as much as 40 dBA. Venting gas or steam to atmosphere is a common process. Examples systems may include backpressure control on a steam header or a controlled, overpressure protection system. A vent system has two potential noise sources that can generate unacceptable, high noise levels: 1) the control valve and 2) the exit point or vent. The control valve, usually inside the building or otherwise in the vicinity of plant employees, will have high noise potential due to the low downstream pressure and the associated high pressure drop. The exit point or vent at the top of the vent stack generates substantial noise as the exiting fluid creates its own jet(s) and turbulence. Controlling the noise from these sources is vital to meeting plant boundary or fence-line noise limits as well as meeting the both regulatory and plant requirements for protection of workers. Valve trim and Diffusers are fluid pressure reduction devices that are typically used to reduce turbulent fluid streams and reduce outlet jet interaction to control noise in process control applications.
For example, typical diffusers are constructed from a hollow housing including a series of passageways throughout the housing walls that connect inlets along the interior surface or inner perimeter to outlets along the exterior surface or outer perimeter of the diffuser. Generally, fluid is admitted into the hollow center of the diffuser and passed through the passageways to the exterior surface. It is understood by one of ordinary skill in the art that conventional diffusers provide noise control by: 1) using multistage pressure reduction within the diffuser housing to divide fluid power between stages and correspondingly reduce acoustic conversion efficiency; 2) shifting the frequency spectrum of the resultant acoustic energy outside the audible range; 3) maintaining exit jet independence to avoid noise regeneration due to jet interaction or coalescence; and 4) managing the velocity of the outlet jets by expanding areas to accommodate the expanding gas. These conventional design techniques address noise issues spanning a broad frequency spectrum. However, it has been discovered that certain fluid pressure reduction applications may experience an additional phenomena resulting from symmetric outlet geometries that yield an undesirable specific tone or peak frequency or multiple peak frequencies.
That is, when outlet geometries are symmetric in area, dimension, and/or location, jets may interact and produce a specific tone or frequency related to the jet interactions under such conditions. Conventional approaches to de-tune or attenuate these tones include decreasing inlet-to-outlet area ratios within the device, reducing the number of inlets available within the device, or adding a baffle around the device. Unfortunately, either such technique may reduce overall fluid capacity of a system or valve. To maintain a given fluid capacity for such a device, the height or the overall diameter of the fluid pressure reduction device must increase. This technique is not viable in certain applications of diffusers or valve trim. For example, increases in stack height or device diameter may make the structure too large to fit within design envelope related to duct work or valve body size and may also be too costly to manufacture. Accordingly, it is desirable to create an improved fluid pressure reduction device that eliminates such objectionable tones or peak frequencies.